Article of manufacture

ABSTRACT

An encapsulated electrical or electronic device having a first layer of electrically insulating material which is then covered by another layer filled with a thermally conductive material such as mesophase pitch based carbon fibers or graphite particles. Both layers are of the same base resin.

BACKGROUND OF THE INVENTION

This application is a continuation-in-part of copending application Ser.No. 07/748,332 filed Aug. 21, 1991 now U.S. Pat. No. 5,236,779, which inturn is a continuation-in-part of application Ser. No. 07/433,819, filedNov. 9, 1989, now abandoned and which in turn is a continuation-in-partof application Ser. No. 07/251,772, filed Oct. 3, 1988, now abandoned.

This invention relates to encapsulated electrical and electronic devicesand, more particularly, it relates to electrical and electronic devicesencapsulated with both an insulating material and a thermally conductivematerial.

Specific techniques for encapsulating electrical and electronic devicesare known as disclosed by Eickman et al. in U.S. Pat. No. 4,632,798.They also disclose that it is common practice to include, within theencapsulating resin, particulate filler material such as silica oralumina which serves to increase the thermal conductivity.

The market segments for electrical devices such as for windings ofmotors, transformers and solenoids and electronic devices such asmicrochips are increasingly moving to miniaturization of such devices.This in turn leads to a rise in internal equipment operating temperatureresulting in not only a need for higher temperature ratings oninsulation materials used for these applications, but also a need formaterials with improved thermal conductivity properties to rapidlyremove the heat generated.

SUMMARY OF THE INVENTION

On order to complement the move toward miniaturization of suchelectrical and electronic devices, an article of manufacture has beendeveloped which comprises an electrical device encapsulated with a layerof an electrically insulating material that in turn is encapsulated witha layer of thermal conductive material forming an outer surface of thearticle. A single phase interface is found between the layers. Thethermal conductive material comprises 10% to 70% by weight andpreferably from about 15% to about 60% carbon fiber, the balance beingmade up of a matrix of resin or a combination of a resin and analternate fiber or filler. The electrically insulating material is madeof a matrix resin or a combination of a resin and a reinforcing fiberand/or a filler. It is important that the resins are the same to enablethe formation of the single phase interface between the layers such thatthe layers appear as one at the interface.

Suitable resinous materials which may be used as the resin include, butare not limited to, polyethylene terephthalate, Teflon® PFA by E. I. duPont de Nemours and Company (DuPont), amorphous copolyamides asdescribed in U.S. Pat. No. 4,681,411, and Hytrel® 7246 by DuPont as wellas thermosetting resins.

The carbon fibers are preferably centrifugally spun from mesophase pitchas disclosed in U.S. Pat. No. 4,861,653 application which isincorporated herein by reference.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic cross-sectional view of a transformer according tothe invention.

FIG. 2 is a photograph enlarged 200× showing the interface between anelectrically insulating material and a thermal conductive encapsulatingmaterial.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

Referring to FIG. 1, the embodiment chosen for purpose of illustrationis an article of manufacture generally designated 10 that includes anelectrical device consisting of a plurality of electrically active wirecoils 12, 14 wrapped around a coil form 16 positioned on a metal core18. The coils 12, 14 serve as the primary and secondary windings of atransformer with connections leading to external terminals (not shown).Surrounding the electrical device is an encapsulating first layer 20 ofan electrically insulating material which in turn is encapsulated with athermally conductive second layer 22 which also can serve as a case forthe transformer.

The thermally conductive layer 22 exhibits a three dimensionalarrangement of fibers within the resin matrix as estimated fromshrinkage data in the x, y, and z coordinate axes directions from moldsize to the final part. More particularly, essentially equal shrinkageof the final part in the x, y, and z directions indicates threedimensional isotropic fiber reinforcement while shrinkage of the finalpart that varies by several orders of magnitude between directionssuggests highly oriented reinforcing fibers.

As best shown in FIG. 2, there is formed a single phase interface 21between first and second layers 20 and 22 such that the layers appear asone at the interface. This single phase interface 21 is created by usingthe same base resin in each layer and molding techniques disclosed inthe following Example. This, in turn, provides for a rapid heat transferfrom layer 20 to layer 22 which could not be obtained with incompatibleor dissimilar resins.

EXAMPLE

In Step I, a non-encapsulated transformer is introduced into a steelinjection molding tool held in a standard injection molding machine. Thecavity of the injection molding tool has (1) notches cut into them so asto allow the transformer leads to protrude from the cavity holding thetransformer and (2) positioning pins to hold the transformer in place sothat the injected polymer can flow completely around the transformer. Acommercial grade DuPont Rynite® FR-530 NC-10 polymer which is a 30 wt. %chopped glass fiber reinforced flame retarded polyethylene terephthlatepolymer is heated to a melt temperature of 280° C. and injected into theclosed steel mold containing the transformer. After two minutes, thetransformer is removed form the original steel mold. step II, a secondsteel mold with similar notches and positioning pins havingapproximately 1/2" to 3/4" larger overall inner dimensions than thefirst mold replaces the first mold in the injection molding machine. A50 wt. % pitch carbon mat fiber reinforced flame-retarded polyethyleneterephthalate polymer made by compression molding alternate layers offlame retarded polyethylene terephthalate polymer film and carbon fiberbats made according to the disclosure in U.S. Pat. No. 4,861,653 whichis subsequently chopped into 1/4" pellets and fed to the injectionmolding machine where it is heated to 280° C. and injected into the toolcavity containing the previously encapsulated transformer of step I.This doubly encapsulated transformer is removed from the mold after 1minute and 40 seconds. A two shot encapsulated transformer was testedfor electrical insulation properties of the inner layer and thermalconductivity properties of the outer layer, according to theUnderwriters Laboratories standard UL 1585, sections Rated SecondaryCurrent Test, and 29, Rated Output Heating Test, and the temperaturerise of 42° C. was much less than the specified maximum temperaturespecified in the UL 1585 standard. The integrity of the electricalinsulation was maintained.

In an alternative embodiment, the carbon fibers described above inconnection with the second layer 22 are replaced with a sufficientloading (up to 60 weight percent) of graphite particles with a particlesize range of from 0.5 microns to 420 microns and preferably less than180 microns. These particles are present in such an amount and orientedwithin a plane of the resin by melting and flowing the resin thatadjacent graphite particles touch each other forming thermal conductivepaths throughout the resin matrix providing improved thermalconductivity as measured by Underwriters Laboratories Standard UL 1585.

What is claimed:
 1. An improved article of manufacture comprising: an electrical device having electrically active components, said components being encapsulated with a first layer of an electrically insulating resin, and a second layer of a thermally conductive material forming the outer surface of said article, said second layer comprising an electrically insulating resin of the same composition as the first electrically insulating resin filled with up to 60 weight percent graphite particles with a particle size range of from 0.5 microns to 420 microns, said graphite particles being oriented within a plane of the resin and present in such an amount that adjacent graphite particles touch each other forming thermal conductive paths throughout the resin matrix providing improved thermal conductivity, there being a single phase interface formed between said first and second layer such that the layers appear as one at the interface.
 2. The improved article of manufacture of claim 1 wherein the particle size is less than 180 microns. 